In a sampling mixer, a digitally modulated signal is sampled by a sampling circuit, and a filter effect is obtained by a switched capacitor incorporated in the sampling circuit (see Patent Document 1 and Patent Document 2, for example).
FIG. 1 is a circuit diagram of sampling mixer 600 described in Patent Document 1, and FIG. 2 is a drawing showing a timing chart of control signals in sampling mixer 600.
In FIG. 1, sampling mixer 600 is provided with TA (transconductance amplifier) 1 that converts a received radio frequency (RF) signal to RF current iRF, in-phase mixer section 2 that samples RF current iRF converted by TA 1, reverse-phase mixer section 3 that is combined therewith, and DCU 4 that generates control signals to in-phase mixer section 2 and reverse-phase mixer section 3.
In-phase mixer section 2 includes sampling switch 5, and Ch (history capacitor) 6 that performs temporally continuous integration of signals sampled by sampling switch 5. In-phase mixer section 2 also includes Cr (rotation capacitors) 7 through 14 that repeat integration and discharge of signals sampled by sampling switch 5, and Cb (buffer capacitor) 15 that buffers signals discharged by Cr 7 through 14.
Furthermore, in-phase mixer section 2 includes damping switch 16 for discharging signals held by Cr 7 through 14 to Cb 15, reset switch 17 that resets signals held by Cr 7 through 14 after signal discharging, and integration switches 18 through 25 for sequentially connecting Cr 7 through 14 to Ch 6. In addition, in-phase mixer section 2 includes discharge switches 26 through 33 for sequentially connecting Cr 7 through 14 to Cb 15, and feedback switches 34 and 35 that control feedback signal input to the sampling mixer 600 side from a DA (digital/analog) converter.
The operation of sampling mixer 600 will now be described, taking the operation of in-phase mixer section 2 as an example.
First, RF current iRF is sampled by sampling switch 5, and becomes temporally discretized discrete signals. These discrete signals are sequentially integrated by Ch 6 and Cr 7 through 14 based on the SV0 through SV7 signals, and undergo filtering and decimation.
By this means, an 8-tap FIR (Finite Impulse Response) filter effect is obtained. The sampling rate at this time is decimated to 1/8. This is because signals held by eight integration switches 18 through 25 are subjected to moving averaging. Such a filter is called a first stage FIR filter. The transfer function of a first stage FIR filter is shown by the following equation.
                    (                  Equation          ⁢                                          ⁢          1                )                                                                      H                      FIR            ⁢                                                  ⁢            1                          =                              1            -                          z                              -                8                                                          1            -                          z                              -                1                                                                        [        1        ]            
Furthermore, since Ch 6 sequentially connected to Cr 7 through 14 holds an output potential, an IIR (Infinite Impulse Response) filter effect is also obtained. Such a filter is called a first stage IIR filter. The transfer function of a first stage IIR filter is shown by the following equation, where Ch is the capacitance value of Ch 6 and Cr is the capacitance value of Cr 7 through 14.
                    (                  Equation          ⁢                                          ⁢          2                )                                                                      H                      IIR            ⁢                                                  ⁢            1                          =                  1                      Ch            +            Cr            -                          Chz                              -                1                                                                        [        2        ]            
Moreover, when an SAZ signal is input to the gates of discharge switches 30 through 33, discharge switches 30 through 33 are turned on while the SAZ signal is high. Then discrete signals integrated by Cr 11 through 14 are simultaneously discharged to Cb 15 via on-state discharge switches 30 through 33.
After this discharging, the D signal goes low, damping switch 16 is turned off, and Cb 15 is disconnected from Cr 11 through 14.
Then the R signal goes high, reset switch 17 is turned on, and signals held by Cr 11 through 14 are reset.
In this way, signals integrated by Cr 11 through 14 are discharged to Cb 15 simultaneously, and a 4-tap FIR filter effect is thereby obtained. The sampling rate at this time is decimated to 1/4. This is because signals integrated by Cr 11 through 14 are subjected to moving averaging by Cb 15.
Signals integrated by Cr 7 through 10 also function in a similar way to Cr 11 through 14. Such a filter is called a second stage FIR filter. The transfer function of a second stage FIR filter is shown by the following equation.
                    (                  Equation          ⁢                                                            ⁢                                                          ⁢          3                )                                                                      H                      FIR            ⁢                                                  ⁢            2                          =                              1            4                    ⁢                                    1              -                              z                                  -                  4                                                                    1              -                              z                                  -                  1                                                                                        [        3        ]            
Four Cr's are connected to Cb 15 in a four Cr 7 through 10 or four Cr 11 through 14 group unit. By this means, an IIR filter effect is also obtained. Such a filter is called a second stage IIR filter. The transfer function of a second stage IIR filter is shown by the following equation, where Cb is the capacitance value of Cb 15.
                    (                  Equation          ⁢                                          ⁢          4                )                                                                      H                      IIR            ⁢                                                  ⁢            2                          =                              4            ⁢                                                  ⁢            Cr                                              4              ⁢                                                          ⁢              Cr                        +            Cb            -                          Cbz                              -                1                                                                        [        4        ]            
Reverse-phase mixer section 3 operates in almost the same way as in-phase mixer section 2, except that sampling is performed 1/2 period later than in the case of in-phase mixer section 2.
When sampling mixer 600 is configured in this way, an output signal of that sampling mixer 600 is a signal that has passed through a first stage FIR filter, first stage IIR filter, second stage FIR filter, and second stage IIR filter, and the overall filter transfer function is given by Equation (1), Equation (2), Equation (3), Equation (4), and the following equation using an equation for current conversion by TA 1, where gm is the transconductance of TA 1 and fRF is the frequency of an input RF signal.
                    (                  Equation          ⁢                                          ⁢          5                )                                                                                                H              =                            ⁢                                                gm                                      π                    ⁢                                                                                  ⁢                                          f                      RF                                                                      ⁢                                  H                                      FIR                    ⁢                                                                                  ⁢                    1                                                  ⁢                                  H                                      IIR                    ⁢                                                                                  ⁢                    1                                                  ⁢                                  H                                      FIR                    ⁢                                                                                  ⁢                    2                                                  ⁢                                  H                                      IIR                    ⁢                                                                                  ⁢                    2                                                                                                                          =                            ⁢                                                gm                                      π                    ⁢                                                                                  ⁢                                          f                      RF                                                                      ⁢                                                      1                    -                                          z                                              -                        8                                                                                                  1                    -                                          z                                              -                        1                                                                                            ⁢                                  1                                                            (                                                                        C                          H                                                +                                                  C                          R                                                                    )                                        -                                                                  C                        H                                            ⁢                                              z                                                  -                          8                                                                                                                    ⁢                                  1                  4                                ⁢                                                      1                    -                                          z                                              -                        32                                                                                                  1                    -                                          z                                              -                        8                                                                                                                                                                                  ⁢                                                4                  ⁢                                                                          ⁢                                      C                    R                                                                                        (                                                                  4                        ⁢                                                                                                  ⁢                                                  C                          R                                                                    +                                              C                        B                                                              )                                    -                                                            C                      B                                        ⁢                                          z                                              -                        32                                                                                                                                                    [        5        ]            
Filter characteristics of the above-described types of filter will now be described with reference to FIG. 3. It will be assumed here that the LO signal frequency is 2.4 GHz, Ch 6 is 15 pF, Cr 7 through 14 are 0.5 pF, Cb 15 is 15 pF, and the transconductance of TA 1 is 7.5 mS.
FIG. 3(a) shows a first stage FIR filter characteristic, FIG. 3(b) shows a first stage IIR filter characteristic, FIG. 3(c) shows a second stage FIR filter characteristic, FIG. 3(d) shows a second stage IIR filter characteristic, and FIG. 3(e) shows an overall filter characteristic of sampling mixer 600. In the conventional-technology characteristic examples in FIG. 3, a signal sampled at 2.4 GHz by sampling switch 5 is output after undergoing 32-decimation. The sampling frequency at this time is 300 MHz, and frequency components separated from the LO frequency in 300 MHz units are folded back to the vicinity of a desired wave. There is a thus problem of a folding frequency appearing in the vicinity of a desired wave if the number of decimations is large.
In particular, when decimation operation is performed in a radio system for UHF band terrestrial digital broadcasting (approximately 470 MHz to 770 MHz) in which the reception band extends over a wide band or the like, folding frequencies appear in the reception band, and there is consequently a demand for a sampling mixer that reduces the number of decimations. Specifically, when terrestrial digital broadcast channel 13 (with a center frequency of approximately 473 MHz) is received by a sampling mixer performing 2-decimation operation, a folding frequency appears every 236.5 MHz from 473 MHz. At this time, 709.5 MHz is within a signal band of terrestrial digital broadcast channel 52 (with a center frequency of approximately 707 MHz), and a channel 52 signal is folded back, causing degradation of reception sensitivity. Therefore, it is necessary for a folding frequency at the time of channel 13 reception by a non-decimation sampling mixer to be made 946 MHz, higher than 770 MHz.
A circuit diagram of conventional sampling mixer 610 operating without decimation is shown here in FIG. 4. In FIG. 4, points of difference from sampling mixer 600 in FIG. 1 are that in-phase mixer section 42 and reverse-phase mixer section 43 are each provided with two Cr's, and DCU 44 output signals are SV0 and SV1 signals, a D signal, an R signal, and FB0 and FB1 signals. FIG. 5 shows a block diagram of DCU 44. DCU 44 is configured using a D flip-flop circuit, which is a general circuit, based on a REF signal necessary for the DCU to generate a control signal. FIG. 6 is a timing chart of sampling mixer 610 control signals. According to FIG. 5 and FIG. 6, the SV0 and SV1 signals are signals obtained by dividing the REF signal by 8, the D signal is a signal obtained by dividing the REF signal by 4, the R signal is one of 4 phased signals based on the REF signal, and the FB0 and FB1 signals are two of 8 phased signals based on the REF signal. When configuring a sampling mixer with a low number of decimations by conventional means as described above, as shown in FIG. 6 a high-frequency REF signal is necessary and control signals with different waveforms (for example, different pulse widths) must be provided.    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-289793 (pp. 6-9, FIG. 3a, FIG. 3b, FIG. 4)    Patent Document 2: US Patent Application Laid-Open No. 2003/0083033 Specification, “SAMPLING MIXER WITH ASYNCHRONOUS CLOCK AND SIGNAL DOMAINS”